This invention relates generally to methods and apparatuses for attaching contacts to a substrate, and more particularly to a method for attaching spherical and/or non-spherical contacts to a substrate, such as a printed circuit or wire board of a wireless test fixture having a grid of mounting pads embedded in its upper surface.
Methods for attaching contacts to a printed circuit or wire board substrate are well-known in the art. One particular method is disclosed in U.S. Pat. No. 5,024,372 to Altman et al. In attempting to attach solder contacts or "bumps", as they are referred to in Altman et al., a thick layer of solder resist material is applied to a printed circuit board substrate wherein some of the solder resist material is selectively removed to provide wells over metallized pads which are fixedly attached to the substrate. Next, solder paste is applied to the substrate in the wells and the excess is removed by a squeegee. The solder bumps are then placed within the wells over the metallized pads and reflowed (e.g., heated) for positively bonding the solder bump to the pad. After bonding the solder bumps to the pads of the substrate, the excess solder resist material is then removed from the substrate. This method of attaching the solder contacts to the substrate is particularly representative of several of the known methods in that it is extremely time consuming to perform since the solder resist material forming the wells surrounding each metallized pad must be applied to and removed from the substrate.
In U.S. Pat. No. 5,439,162 to George et al., there is disclosed a method of attaching an integrated circuit to a printed circuit board. The method disclosed in George et al. includes the steps of depositing solder flux on contacts which are already attached to an integrated circuit, depositing solder on the contacts, engaging the contacts with the mounting pads of the printed circuit board, and heating the contacts to solder them to the pads. One problem associated with this method is that it is directed to attaching an integrated circuit having solder contacts or bumps already formed thereon with a printed circuit board rather than a method of attaching the contacts to a substrate.
U.S. Pat. No. 5,284,287 to Wilson et al. is directed to a method of attaching spherical contacts to a substrate which utilizes a tool having a vacuum mechanism for securing the contacts to the tool. More specifically, the pick-up tool has cavities which are arranged to correspond with mounting pads of the substrate. The contacts are held in place on the tool by the vacuum and are lowered into flux so that a dab of flux material is applied to each contact. The tool is then moved so as to position the contacts over the substrate, and releases the contacts from the tool onto the mounting pads of the substrate. The substrate is heated to solder the contacts thereto. This method, while being effective in attaching solder contacts to the substrate, requires complicated machinery and is expensive to implement.
The U.S. Pat. No. 5,118,027 to Braun et al. and U.S. Pat. No. 5,147,084 to Behun et al. are further representative of the prior art in this area. It should be noted that there is presently a need in the industry for a method which conveniently, expeditiously and cost-efficiently attaches spherical and non-spherical contacts onto mounting pads of the substrate.
The method of the present invention is directed to attaching spherical and/or non-spherical contacts to a substrate having mounting pads arranged in a predetermined array. The method comprises the steps of positioning the substrate over a fixture constructed and arranged for supporting the substrate thereon, and positioning a stencil over the substrate. The stencil has openings formed therein in the predetermined array, and is held and aligned by the fixture in such a position that the openings of the stencil are aligned with the mounting pads of the substrate. Soldering paste is applied onto the stencil so as to fill the openings of the stencil wherein a dab of paste is left in the openings of the stencil. The stencil is removed from the substrate so as to leave the dab of paste over each mounting pad, and a contact loading plate is positioned over the substrate. The contact loading plate has openings formed therein in the predetermined array, and is held and aligned by the fixture in such a position that the openings of the plate are aligned with the mounting pads of the substrate. The openings of the contact loading plate are then filled with contacts, the arrangement being such that the contacts engage the dabs of paste positioned over the mounting pads. Next, the contact loading plate is removed from the substrate and fixture so as to leave the contacts on the dabs of paste provided on the mounting pads of the substrate. The substrate is then removed from the fixture, and heat is applied to the substrate so as to solder the contacts to the mounting pads.
An apparatus for achieving the method of the present invention comprises a fixture, and a stencil having openings formed therein in the predetermined array. The stencil is held and aligned by the fixture in such a position that the openings of the stencil are aligned with the mounting pads of the substrate. The stencil is adapted to receive thereon soldering paste so as to fill the openings of the stencil so that a dab of paste is left in the openings of the stencil. The apparatus further includes a contact loading plate having openings formed therein in the predetermined array. The contact loading plate is held and aligned by the fixture in such a position that the openings of the plate are aligned with the mounting pads of the substrate. The openings of the contact loading plate are adapted to receive contacts therein wherein the contacts engage the dabs of paste positioned over the mounting pads.
Accordingly, among the several objects of the present invention are the provision of an improved method which is capable of accurately attaching spherical and non-spherical contacts to mounting pads of a substrate; the provision of such a method which is simple to follow and easy to perform; the provision of such a method which is capable of depositing a relatively flat, disc-shaped solder preform onto the mounting pads of the substrate; the provision of such a method which incorporates easy to use and low cost equipment; the provision of such a method which reduces the amount of time normally required to solder the contacts onto the mounting pads of the substrate; and the provision of such a method which produces high-quality results.
Also among the several objectives of the present invention is that of an apparatus which achieves each of the objects of the method.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.